Adaptation of a methanogenic consortium to arsenite inhibition

Arsenic (As) is a ubiquitous metalloid known for its adverse effects to human health. Microorganisms are also impacted by As toxicity, including methanogenic archaea, which can affect the performance of a process in which biological activity is required (i.e., stabilization of activated sludge in wastewater treatment plants). The novel ability of a mixed methanogenic granular sludge consortium to adapt to the inhibitory effect of arsenic As was investigated by exposing the culture to approximately 0.92 mM of arsenite (As^III) for 160 days in an arsenate (As^V)-reducing bioreactor using ethanol as the electron donor. The results of shaken batch bioassays indicated that the original, unexposed sludge was severely inhibited by As^III as evidenced by the low 50 % inhibition concentrations (IC₅₀) determined, i.e., 19 and 90 μM As^III for acetoclastic and hydrogenotrophic methanogenesis, respectively. The tolerance of the acetoclastic and hydrogenotrophic methanogens in the sludge to As^III increased 47-fold (IC₅₀ = 910 μM) and 12-fold (IC₅₀ = 1100 μM), respectively, upon long-term exposure to As. In conclusion, the methanogenic community in the granular sludge demonstrated a considerable ability to adapt to the severe inhibitory effects of As after a prolonged exposure period.